Flotation machines are known from before, e.g., from patent specifications FI 78628 B, FI 115612 B, WO 93/14876 for floating hydrophobic particles from aqueous slurry that contains these particles. Typically, the flotation machine includes a flotation cell, which comprises a vertical side wall and a bottom and, possibly, a cover. The flotation machine comprises one or more air distribution and mixing devices, which are arranged in the middle part of the cell for distributing air to the slurry to form froth an to mix the slurry in the flotation cell. The middle part of the cell refers to the location essentially at the vertical centre line of the cell. In the case of several air distribution and mixing devices, they are located symmetrically around the centre line in its vicinity. The air distribution and mixing device includes a rotor part that comprises air distribution apertures. A drive shaft is arranged to extend vertically in the flotation cell. The rotor part is attached to the lower end of the drive shaft. The drive shaft comprises a hollow interior, which constitutes a flow channel for conducting flotation air to the air distribution holes of the rotor part.
In prior art, the operation of the drive shaft is arranged by means of a high-speed electric motor, such as a conventional cage induction motor, which drives the drive shaft through a gear box or a V-belt drive. To fit the drive shaft with a bearing, a separate support bearing unit is arranged in connection with the V-belt drive, being supported by the overhead support structure of the flotation cell. Air supply into the hollow drive shaft is arranged through the support bearing unit in the manner described, e.g., in the specification FI 115612.
A problem with the prior art is that the belt drive or gear box reduces the efficiency of the power transmission. A further problem is the large space utilization in terms of height required by the transmission structure. The belt transmission system, which comprises large-size belt pulleys, requires a large-size encapsulation around the same. The V-belt drive also induces high internal stresses in the support structures. There is a further problem that the present combination of the electric motor and V-belt drive or the gear box and bearing unit has a complex structure and it contains many parts that are susceptible to wearing and failure, reducing the reliability and causing a repeated need of maintenance. The abrasive dust emitted from the flotation process causes wearing of the V belt and the V-belt drive pulleys. Furthermore, the operation and transmission equipment is heavy and it requires a sturdy supporting structure, which incurs costs. The air supply into the hollow drive shaft is difficult to arrange.